This invention relates to plasticized polyvinyl butyral sheet for safety glazing and more particularly to a melt shaping process therefor.
Plasticized polyvinyl butyral sheet is very well known as an interlayer for use with glass or rigid plastic panels in laminated safety glazing assemblies. The polyvinyl butyral resin is more particularly partial polyvinyl butyral insofar as containing about 15 to about 30 percent by weight residual hydroxyl groups which promote adherence of the sheet to the glass or plastic. Such polyvinyl butyral or partial polyvinyl butyral, (interchangeably referred to as PVB), is conventionally prepared by partially acetalizing polyvinyl alcohol (PVOH) with butyraldehyde. The PVB is then conventionally mixed with plasticizer and melt processed into sheeting which is usually collected and stored in roll form before use. In use, sections for individual glazing units are cut from the roll, placed between or adjacent one or more laminating panels and the sandwich pressed together in an autoclave at elevated temperature and pressure to form the laminate.
Plasticized PVB sheet used in this way inherently tends to stick to itself (sometimes called "blocking") at ambient temperatures typically encountered during storage before laminating. Expensive precautions have been taken to prevent this. For example, the sheet has been transported and stored at low refrigeration temperatures, or interleaved with polyethylene film or dusted between facing layers with sodium bicarbonate. It has been and continues to be very desirable to alleviate this blocking problem associated with plasticized PVB sheet.
Another problem occurs during laminating when incomplete contact exists between the PVB sheet and the contiguous laminating panel. This is caused by thickness tolerance variations in the sheet and laminating panel and results in visually apparent, local, unlaminated patches appearing as bubbles. In the past, when these patches appeared on laminating lines the problem was overcome by extending the autoclave cycle to heat the sheet to a higher temperature to promote greater polymer flow. But this undesirably results in increased energy costs and imposes a capacity-affecting limitation on laminating lines. Tailoring the sheet polymer modulus to promote slight additional flow into an uneven area of the gap between the sheet and opposing panel could alleviate the problem. But this works at cross purposes with minimizing sticking of the sheet to itself before laminating. In other words, reducing modulus to promote flow during laminating will undesirably increase stickiness at ambient temperature. On the other hand, increasing ambient temperature modulus to stiffen the sheet enough to resist sticking will decrease flow during laminating. Yet optimum flow during laminating is increasingly important in vehicle glazing applications as the amount of glass per vehicle is increased.
It would be very desirable to simultaneously favorably influence each of these problem areas associated with use of plasticized PVB sheet in laminated safety glazings and avoid performance trade-offs which have occurred in reducing one problem at the expense of the other. To the best of our knowledge, however, such corrections have not been provided in the prior art.